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Chloroplast DNA nucleotide sequences provided the first
molecular evidence bearing on the origin and relationships of the Hawaiian silversword
alliance. These data support the conclusion that the Hawaiian silversword alliance is a
monophyletic group derived from ancestors very similar to the extant Pacific coast
tarweeds Anisocarpus scabridus (Eastwood) B. G. Baldwin [Raillardiopsis scabrida
(Eastw.) Rydberg], Kyhosia bolanderi (A. Gray) B. G. Baldwin [Madia bolanderi (A. Gray) A.
Gray], and Carlquistia muirii (A. Gray) B. G. Baldwin [Raillardiopsis muirii
(A.
Gray) Rydberg]. |
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Carlquistia muirii is a rhizomatous, mat-forming plant with herbaceous
stems 2-3 dm tall that bear mostly alternate leaves measuring 1--3 cm long by 1.5--4 mm
wide. The species occurs only in the southern Sierra Nevada and Santa Lucia Range of
California, on open granitic slopes at elevations of 1,100--2,500 m. The chromosome number
is 2n = 16. |
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Anisocarpus scabridus, California, Lake County, ridge E of Hull Mt. summit.
The chromosome number is 2n = 14. |
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Kyhosia bolanderi has fleshy rootstocks that bear herbaceous stems 5--12 dm
tall. The linear leaves are clustered and opposite toward the base, sparse and alternate
above. The heads are large and radiate. The species occurs in the Sierra Nevada and
mountain ranges of northern California and adjacent Oregon, in meadows and on stream banks
at elevations 1,100--2,600 m. The chromosome number is 2n = 12. |
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The molecular data encouraged an attempt to artificially hybridize Dubautia laevigata, and Carlquistia muirii. The
cross was very successful, and soon several vigorous intergeneric, trans-Pacific hybrids
were in cultivation. The female parent, Dubautia laevigata is in the upper left,
the male parent, Carlquistia muirii, on the right, and the intergeneric hybrid
is in the center. The middle illustration shows 22 chromosomes in a somatic cell of an
individual of this hybrid combination. The lower illustration shows two cells at meiotic
metaphase I. The cell on the left has 22 unpaired chromosomes while the cell on the right
has 16 unpaired chromosomes and 3 apparent bivalents. The arrows indicate two partially
overlapping univalents. Almost a third of the microsporocytes underwent a mitotic division
at a time when reductional division was expected. About 45% of the cells observed had 1--3
pairs of chromosomes. The maximum number of pairs of chromosomes observed (4) was seen in
only one cell (0.3%). Pollen stainability in this hybrid was 49.3 %. Nearly all of the
stained grains were tetracolporate (compared to normal tricolporate grains), and were
larger than normal, indicating their probable diploid condition. |
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Artificial hybridization of Dubautia with Kyhosia bolanderi also
proved easy to accomplish. The upper illustration shows the female parent on the left,
which was itself a fully fertile hybrid between Dubautia knudsenii
and D. laxa. On the right is the male parent,
Kyhosia bolanderi, and in the center, the intergeneric hybrid. The hybrid was vigorous, and
as expected, exhibited 2n = 20 chromosomes. The cell on the left of the lower illustration
shows 20 unpaired chromosomes. The 6 largest chromosomes are from Kyhosia
while the
remaining 14 smaller chromosomes are from Dubautia. An arrow indicates two
contiguous univalents, one somewhat distorted. The cell on the right shows 3 apparent
bivalents, a,b, and c. Considering the size of the chromosomes, the bivalents labeled a
and b appear to represent infragenomic pairing of Dubautia chromosomes. Although
about 80% of the microsporocytes had from 1--4 bivalents, the chromosome associations
appeared to be looser than in the previous hybrid combination. Pollen stainability was
about 4.4% in this hybrid. |
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Given that Kyhosia bolanderi (n = 6) and Carlquistia
muirii (n
= 8) each produce vigorous artificial hybrids with Dubautia (n = 14), it
is tempting to speculate that the genome of the Hawaiian silversword alliance arose
through hybridization of such diploid species followed by chromosome doubling (i.e.,
allopolyploidy). The upper illustration on the left summarizes this hypothetical scheme.
The diploid hybrid between Kyhosia bolanderi and Carlquistia muirii
that could represent the synthesis of the Hawaiian genome has been produced artificially.
The parental species are illustrated in the figures above while the intergeneric hybrid
appears at the upper right in the next entry. The bottom illustration on the left shows a
microsporocyte from this hybrid at meiotic metaphase I with 8 unpaired chromosomes and 4
bivalents. Microsporocytes from this hybrid commonly underwent a mitotic division at the
time when reductional division was expected. This resulted in as high as 40% stainable
pollen grains that were tetracolporate, larger than normal, and presumably diploid. These
grains proved to be functional in the production of the trigeneric hybrid described below. |
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In a continuing attempt to assess chromosome pairing and the genetic compatibility of
these genomes, pollen from the artificial hybrid between Kyhosia bolanderi
and Carlquistia muirii (top right) was transferred to stigmas of Dubautia
scabra (top left). Amazingly, the first attempt resulted in three vigorous
individuals of a spectacular trigeneric hybrid (top center) that combines the diploid
genomes of two mainland genera with the tetraploid genome of the Hawaiian silversword
alliance. This hybrid had low chromosome pairing with about 80% of the microsporocytes
showing 1--4 pairs in addition to unpaired chromosomes. The cell illustrated (left, below)
has one pair and 26 univalents. The largest chromosomes are from Kyhosia, the
intermediate-sized ones from Carlquistia, and smallest ones from Dubautia.
The remaining cells had all univalents or underwent mitosis at the time reductional
division was expected. About 11.5% of the pollen grains were stainable and these were
mostly tetracolporate and larger than normal. The low chromosome pairing in this hybrid is
not surprising considering the low amount of pairing in the primary hybrids and given the
fact that both the mainland tarweeds and the members of the Hawaiian silversword alliance
have been undergoing rapid, independent chromosome evolution during the estimated 6
million years of separation of these groups. |
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The molecular evidence linking Anisocarpus scabridus (n = 7) to
the Hawaiian silversword alliance suggests another alternative to the allotetraploid origin
of the Hawaiian genome outlined above. In this scenario the Hawaiian genome (n =
14) could have originated directly by autopolyploidy from such a diploid ancestor
(illustration at left). The artificial hybrid between Dubautia and Anisocarpus
scabridus that may have helped to assess this hypothetical origin of the Hawaiian
genome was successfully produced but was not vigorous and died before reaching
reproductive maturity. |
